1. Field of the Invention
The present invention relates generally to a vibration sensor for an automotive vehicle which can detect mechanical vibration generated from an internal combustion engine, for example, such as knocking, and more specifically to a vibration sensor for an automotive vehicle having a multi-peak vibration characteristic of wider responsiveness and of higher S/N ratio.
2. Description of the Prior Art
Generally, if strong engine knocking continues for a long time while the internal combustion engine is operating, the knocking will exert an undesired influence on the engine in durability or performance. However, it has also been known that the best engine operating conditions with respect to engine torque characteristics and fuel consumption rate can be obtained when the engine operates with a weak knocking at relatively slow engine speeds.
Heretofore, various systems have been proposed which can detect engine knocking conditions and regulate the engine spark timing according to the knocking conditions to maintain a weak knocking state at all times to improve the engine torque characteristics and fuel consumption rate. This is because engine spark timing has a close relationship to engine knocking conditions.
In the system mentioned above, a vibration sensor is indispensable in order to detect the knocking conditions of an internal combustion engine and, therefore, there has already been proposed a vibration sensor for an automotive vehicle having a piezoelectric vibration element mounted therein in cantilever fashion. However, since a single vibration element with a relatively sharp response characteristic has conventionally been used for this vibration sensor, the problem has occurred that if knocking frequency deviates from the resonance frequency, it is impossible to detect knocking vibration sufficiently and correctly and the S/N ratio of the sensor drops sharply.